1. Field of the Invention
This invention relates to a controllable piezoelectric shear actuating device and more specifically to a device which effects motion in an object through the imposition of traction and release forces thereon.
2. Background Art
A few devices incorporating piezoelectric assemblies for actuation of same are known. Rev. Sci. Instrum. 58 (1), January 1987, details a dynamic piezoelectric translation device whose motion is caused by a sawtoothlike periodic acceleration of a piezoelectric element and the inertia of the translation stage.
Another device incorporating a piezoelectric linear actuator known as the Inchworm.sup..TM. (U.S. Pats. Nos. 3,902,084 and 3,902,085) describe an assembly of extension mode piezoelectric tubular elements wherein alternate gripping and releasing of a coaxial rod by end tubes and timely stretching and shrinking of a center tube effect translation of the rod. More specifically, the Inchworm (Trademark of Burleigh Instruments, Inc., New York) achieves positioning by cyclically releasing, extending, gripping, releasing, contracting, and gripping a rod-like object. The rod-like object is moved along one axis. The use of one extending and two gripping portions does not permit motion of the rod-like object with constant velocity. The thickness piezoelectric deformation is used in the gripping portions. The extensional piezoelectric deformation is used in the extension and contracting portion. During the releasing portions of a cycle the gripping normal force is reduced to zero. Cyclical gripping and releasing causes cyclical conversion of stored elastic potential energy into kinetic energy and the converse. The cyclic energy conversion acoustically and dynamically excites the rod-like object, the piezoelectric actuators, and all structures adjunct thereto. The position of the rod-like object changes with changing temperature. Rubbing of the rod-like object occurs at the beginning and at the end of each gripping and releasing portion of the cycle except when the rod-like object has zero velocity. The efficiency of the conversion of electrical to mechanical energy is limited by rubbing. The useful life of the surface portions of the Inchworm which are in contact with the rod-like object is limited by rubbing. The extension portion of the Inchworm is subjected to tensile stresses during the contracting portion of each cycle. The tensile stresses bound the range of the combination of moving mass, contracting force, and cycling frequency. The gripping stroke of the thickness deforming piezoelectric portion limits the size of the force acting radially on the rod-like object at which cycling is effective. The last position of the rod-like object attained by the Inchworm is lost when electrical charge is removed from the piezoelectric portions because the rod-like object is not restrained. During normal operation the Inchworm cannot deter rotation of the rod-like object about its axis.
In U.S. Pat. No. 4,775,815, the motional portion of a motor linearly translates in one direction. The motional portion is affixed to a movable object by a mechanical linkage. The range of the translation of the movable object is not greater than the range of the translation of the motional portion derived from piezoelectric stroke in combination with any leverage of the mechanical linkage. The piezoelectric stroke is increased by increasing the height of the layered actuator body portion. Increasing the height of the body portion increases the electrical capacitance. Increasing the capacitance decreases translation speed at a given force and range of stroke, and decreases the force with a given speed and range of stroke, all other elements being constant. The height of the piezoelectric body changes as the temperature of the body changes. During translation the piezoelectric body is subjected to tension stresses due to bending and shear stresses due to forces applied to or discovered at the motional portion, said stresses imposing a limit on the combination of speed, stroke and force. The broad surfaces of piezoelectric layers are electrically insulated each from the others. Since the cross section of the piezoelectric body is constant between the motional portion and the mounting base, the mass near the motional portion is also translated; the inertial reaction force of said mass adds to the force applied by the movable object and the adjunct mechanical linkage, thereby bounding the range of the combination of speed, stroke and force.
These piezoelectric actuators are readily distinguished from the present invention in that, for example, in the first instance to create a translational motion, a sawtooth electrical wave form must be applied to a piezo tube in order to overcome the static friction limit within the device thereby evidencing the principle of inertial sliding of a platform. With the Inchworm assembly the translation is intermittent due to grip, release, and the gripping force is unloaded twice for every step.
As described in more detail below, the piezoelectric shear device or actuator of the present invention provides for a translation which is both smooth and continuous, axial loads maintain piezoelectric compression, and normal force being constant absents vibrational interference which is encountered in the aforementioned piezoelectric devices.